The present invention is directed, in general, to analysis of monolithic structures and, more specifically, to a scanning electron microscope/energy dispersive spectroscopy (SEM/EDS) sample preparation method and a monolithic sample produced by way of the method.
In semiconductor processing today it is often necessary to spectroscopically examine portions of a semiconductor die to determine the results of new or conventional processes. The examination may be to confirm the results of an experimental process, or even to determine the nature of a particular failure or defect in a semiconductor device. Of course, because of the nature of integrated circuits, the examination must often be performed on samples cut from the die in question. Scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) is frequently used in the determination of the composition of target material in a feature of a semiconductor die. In SEM/EDS, X-rays generated by the primary electron beam""s interaction with the target assist in determining the composition of the target.
Referring initially to FIG. 1, illustrated is a cross-sectional view of a conventional SEM sample symbolically showing effects of the SEM process. A primary electron beam 100 with width 180 is directed to a sample 110 having a thickness 111. The sample 110 may be cut from any orientation within a material of interest. In this embodiment, the beam 100 has a width 180 and a tear shaped interaction volume 120, with depth 160. For example, at a given beam energy and in a given material, a beam width 180 of 10 nm can produce an interaction volume with a maximum width 170 of 1 xcexcm. The beam 100 interacts with essentially all of the material in the interaction volume 120, producing secondary electrons 130, backscattered electrons 140, and X-rays 150. The specific X-rays present in the spectra are traceable by EDS to particular elements present at the site of the SEM examination.
As device sizes are made ever smaller, a defect that was insignificant in the past becomes quite significant. However, as the semiconductor features and defects become smaller, it has become increasingly difficult to obtain the lateral resolution needed from EDS for compositional identification of small defects and thin layers. With a decrease in feature size, the feature becomes a smaller percentage of the target, thereby reducing the X-ray emanations from the feature. It is well known that the interaction volume 120 depth 160 and width 170 are dependent upon the accelerating energy of the primary electron beam 100. However, because the beam 100 interacts with the entire interaction volume 120, precise data on the nature of a feature of interest 160 at the examination site may be unclear. In order to reduce the interaction volume, the primary beam accelerating voltage has been reduced to account for the reduced feature size. However, several elements commonly used in semiconductor fabrication have overlapping peaks at lower beam energies, and the ability to distinguish between the elements is therefore lost at these low beam energies. At higher beam voltages more peaks in the EDS spectra allow for better discrimination between the elements present. Therefore, reducing beam energy does not effectively improve element discrimination. Of course, tool manufacturers have worked to improve the sensitivity of their tools. Unfortunately, these efforts have met with limited success, and the available sensitivity is still not sufficient for current and projected needs.
Referring now to FIG. 2A, illustrated are EDS spectra of a conventional semiconductor SEM sample taken with an incident electron beam of 5 keV. As can be seen, the results make it difficult to resolve between oxygen 210 and titanium 220. There also appears to be copper 230 present, as well as aluminum 240, silicon 250 and phosphorous 260. Referring now to FIG. 2B, illustrated are EDS spectra of the same area as shown in FIG. 2A with an incident electron beam of 10 keV. Comparing FIG. 2B to FIG. 2A, it is clear that oxygen 211, titanium 221, aluminum 241, silicon 251 and phosphorus 261 are all present. It is also now evident that copper is not present. It is readily apparent that the definition of the peaks 211, 221, 241, 251, 261 is much better at the higher (10 keV) beam energy.
Referring now to FIG. 3A, illustrated are EDS spectra taken at 5 keV of a different semiconductor SEM sample. At this voltage, titanium 310 and possibly silicon 320 appear to be present. However, one who is skilled in the art will readily observe because of the relative magnitudes of the peaks at X-ray energy levels of 0.6 keV and above that it is extremely difficult to discern the presence of any particular element. Referring now to FIG. 3B, illustrated are EDS spectra taken at 10 keV of the same area as shown in FIG. 3A. One who is skilled in the art will readily appreciate that the spectra of FIG. 3B clearly allows one to differentiate peaks associated with titanium 311, silicon 321, and tungsten 331. Comparing FIG. 3B to FIG. 3A, one can readily understand that the presence of tungsten is simply not recognizable from the spectra of FIG. 3A. Thus higher beam energies make identification of elements much easier.
Accordingly, what is needed in the art is a sample preparation method that minimizes the interaction volume while retaining the feature of interest.
To address the above-discussed deficiencies of the prior art, the present invention provides a method of preparing a monolithic structure for scanning electron microscope/energy dispersive spectroscopy (SEM/EDS) and a sample produced by way of the method. In one embodiment, the method includes: (1) aiming a focused ion beam at any location beneath an area of interest of any size or depth in the monolithic structure and (2) employing the focused ion beam to remove at least a portion of an interaction volume of material beneath the area of interest. The area of interest preferably remains substantially intact for the subsequent spectroscopy.
The present invention therefore introduces the broad concept of employing a focused ion beam to remove at least some of the interaction volume beneath an area of interest that would otherwise generate unwanted background noise during subsequent SEM/EDS analysis. For purposes of the present invention, a xe2x80x9cmonolithic structurexe2x80x9d is defined as a structure formed in or on a body of material. A monolithic structure may be a semiconductor wafer (such as a silicon or gallium arsenide wafer), but can be composed of any material. For purposes of the present invention, an xe2x80x9carea of interestxe2x80x9d is defined as any portion of the monolithic structure that is to be the subject of analysis.
In one embodiment of the present invention, the area of interest is a portion of a surface of the monolithic structure. Alternatively, the area of interest could be located above or below the surface of the monolithic structure.
A preferred embodiment of the present invention comprises employing the focused ion beam to remove an entirety of the interaction volume. Of course, removal of only a portion of the interaction volume may be advantageous in some applications.
In one embodiment of the present invention, the employing comprises employing the focused ion beam to remove at least the portion of the interaction volume such that the area of interest becomes membranous. The membranous area of interest may, but need not, have a thickness comparable to samples employed in conventional transmission electron microscope (TEM)/EDS analysis.
In one embodiment of the present invention, the monolithic structure comprises a silicon wafer. Alternatively, the monolithic structure could be a gallium arsenide wafer or a wafer or body of another suitable composition.
In one embodiment of the present invention, the monolithic structure comprises at least one layer located on a silicon substrate. Alternatively, the monolithic structure could comprise a layer formed within the silicon substrate, or the substrate itself.
In one embodiment of the present invention, the method further includes performing the energy dispersive spectroscopy on the area of interest at an energy greater than 5 keV. In an embodiment to be illustrated and described, the energy dispersive spectroscopy is performed at an energy of about 10 keV. Those skilled in the pertinent art will understand, however, that the present invention is not limited to a particular energy level or range thereof.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.